1. Field of the Invention
The present invention relates to a vehicle generator control system which controls field current to be supplied to a field coil of a generator so as to regulate the output voltage of the generator.
2. Description of the Related Art
Recently, there is a demand for increasing output power of a vehicle generator although torque for driving electric accessories has been reduced to improve gas mileage. Accordingly, engine idling rotation may change widely if the output power of the vehicle generator changes. In order to prevent the above problem, the engine is controlled according to data on the input torque of the generator, such as the field current. Such data are transmitted from the generator to an engine control system, which controls the engine and the generator according to a vehicle operating condition.
U.S. Pat. No. 5,140,253 or its counterpart JP-A-2-184300, U.S. Pat. No. 5,448,154A or its counterpart JP-B-2916837 and JP-A-2-307400 disclose vehicle generator control systems which include a field current drive transistor for controlling the field current of the vehicle generator to be lower than a predetermined current value. In such generator control systems, the field current flowing through the drive transistor is detected, so that the drive transistor is turned off if an amount of the field current exceeds a limit current value. Therefore, sharp increase in the input torque of the generator can be prevented even if electric load of the generator abruptly increases. Thus, smooth operation of the engine can be ensured.
In the vehicle generator control system disclosed in JP-A-2-184300, the field current driving transistor is turned off for a fixed period if the field current exceeds a limit current value even in an instant. This does not properly control the field current. The field current driving transistor of this control system is not controlled at a fixed cycle. Therefore, temperature rise caused by switching loss of the drive transistor increases if the switching cycle is very short. On the other hand, the output voltage of the vehicle generator changes widely if the switching cycle is very long.
Further, it is not possible to detect the amount of the current flowing through a flywheel diode when the drive transistor is turned off. If the field current driving transistor is turned off for a long time, the amount of the field current becomes zero. However, the driving transistor is controlled as if the amount of the field current is the same as the field current right before the transistor is turned off. As a result, a big difference in the input torque of the vehicle generator from the fact may arise, resulting in undesired increase in engine rotation.
For example, in a conventional vehicle generator control system, the field current driving transistor is turned on or off to regulate the output voltage of a vehicle generator to a regulation voltage when an electric load is connected. If the electric load is disconnected from the generator thereafter, the output voltage of the generator increases to be higher than the regulation voltage Vreg until the field current is reduced. In the meanwhile, the field current driving transistor is turned off. Because the field current can be detected only while the field current driving transistor is turning on, the conventional generator control system carries out its control according to the field current before the electric load is disconnected from the generator. Because the field current flowing through the field coil decreases according to the time constant of the field coil, there is a difference ΔIF in the detected current value from the actual current value, as shown in FIG. 9. This difference causes a temporary increase in engine rotation when the engine is idling.
In the vehicle generator control system disclosed in JP-B-2916837 or JP-A-2-307400, the field current flowing through the field driving transistor is smoothed in order to detect an amount of the field current. It is not possible to take the current flowing through the flywheel diode into account.